The present invention relates generally to radiocommunication systems and, more particularly, to techniques and structures for the efficient use of packet data communications in radiocommunication systems.
The growth of commercial communication systems and, in particular, the explosive growth of cellular radiotelephone systems worldwide, has compelled system designers to search for ways to increase system capacity and flexibility without reducing communication quality beyond consumer tolerance thresholds. For example, most early cellular communication systems provided services using circuit-switched technologies. Now, however, mobile calls may be routed in a circuit-switched fashion, a packet-switched fashion, or some hybrid thereof. Moreover, it has become increasingly desirable to couple and integrate mobile cellular telephone networks, for instance a GSM network, to Internet protocol (IP) networks for call routing purposes. The routing of voice calls over IP networks is frequently termed xe2x80x9cvoice over IPxe2x80x9d or, more succinctly, VoIP.
Packet-switched technology, which may be connection-oriented (e.g., X.25) or xe2x80x9cconnectionlessxe2x80x9d as in IP, does not require the set-up and tear-down of a physical connection, which is a significant difference relative to circuit-switched technology. This feature of packet data typically reduces the data latency and increases the efficiency of a channel in handling relatively short, bursty, or interactive transactions. A connectionless packet-switched network distributes the routing functions to multiple routing sites, thereby avoiding possible traffic bottlenecks that could occur when using a central switching hub. Data is xe2x80x9cpacketizedxe2x80x9d with the appropriate end-system addressing and then transmitted in independent units along the data path. Intermediate systems, sometimes called xe2x80x9crouters,xe2x80x9d are stationed between the communicating end-systems to make decisions about the most appropriate route to take on a per packet basis. Routing decisions are based on a number of characteristics, including, for example: least-cost route or cost metric; capacity of the link; number of packets waiting for transmission; security requirements for the link; and intermediate system (node) operational status.
In packet data communication schemes, access to the system is provided on a random basis using a packet data scheduler disposed in the fixed part of the system. For example, a mobile station carries out a random access within a cellular digital packet data communication system in order to initiate a data transfer session. The random access can be carried out, however, only when the scheduler announces an idle time slot in the downlink. Then the mobile station initiates a transfer (for example by transmitting a BEGIN frame) in the idle time slot. When the cellular digital packet data communication system receives the transfer, it acknowledges receipt of the BEGIN frame to the specific mobile station. This acknowledgment indicates to the mobile station that it has been activated and will later be scheduled to transfer data with the system. If the cellular digital packet data communication system did not receive the initial transfer due to, for example, a collision of packets received from several mobile stations attempting random access at the same time, then the mobiles will retry random access after waiting for some e.g., random, time period.
Once a mobile station has made a successful random access, and is therefore active, it is scheduled by the system to transfer packets on a radio channel. The scheduling can be carried out on basis of the mobile""s Quality of Service (QoS) or other widely known methods. With the introduction of new services or applications over packet data systems, for example, real time (RT) services such as VoIP, there will be a large variety of Quality of Service (QoS) demands on the network. Certain users, for example, those utilizing real time voice applications will have a very high demand for the availability of transmission resources, whereas users, for example, who transmit short messages or electronic mail, will be satisfied with a lower availability of transmission resources.
For example, in a UMTS system, there are four proposed QoS classes: the conversational class; streaming class; interactive class; and background class. The main distinguishing factor between these classes is the sensitivity to delay of the traffic. Conversational class traffic is intended for traffic which is very delay sensitive while background class traffic is the most delay insensitive traffic class. Conversational and streaming classes are intended to be used to carry RT traffic flows whereas interactive and background classes are intended to be used to carry Internet applications (e.g., WWW, E-mail, Telnet, FTP, etc.).
In considering how to accommodate varying QoS requirements in wireless packet data systems, delay constraints are one consideration. For example, a user""s subscription may specify a QoS parameter which indicates that, for a particular application, packet delay can be no more than a predetermined number of milliseconds per packet. The packet data system will, under those circumstances, need to have tools to be able to monitor and influence packet delay to provide service at different QoS levels. In wireless packet data systems, channel throughput (and thereby delay) can be adjusted by selecting, for example, an appropriate modulation and coding scheme for a given link quality between the mobile station and the wireless packet data system. An idealized relationship between channel throughput and radio link quality (as represented by the carrier-to-interference (C/I) ratio) is depicted in FIG. 1.
Power control is another technique which may be used to adjust the operation of a radiocommunication system. In conventional, circuit-switched radiocommunication systems, power control was used in conjunction with C/I targets to ramp transmit power up or down for each link in a manner that was intended to globally control link quality. However, in wireless packet data systems, power control becomes more complicated since a single C/I target for all radio links does not exist. Link quality is associated with throughput and, therefore, in order to improve the link quality for a first user who is experiencing poor link quality, it is necessary to lower the quality of a second user""s link. From a time perspective, once the second user""s link is reduced in quality to improve the quality of the first user""s link, it takes longer to transmit a sequence of packets to the second user, e.g., due to the increased need to retransmit packets over the second user""s link.
Applicants have discovered that part of the problem with trying to apply conventional power control techniques to packet data systems is that they fail to adapt transmit power as a function of time. In particular, packet data systems have a total transmit delay which is a function of queue delay (Dq) which is the amount of time that a packet spends being buffered prior to transmission, retransmission delay (Dr) which is the amount of additional time needed to retransmit a packet or portion of a packet that was improperly received, as well as the transmit delay (Dt), which is the data rate depending on the selected modulation and/or coding scheme. Dt can be reduced if a higher level modulation and/or a reduced error coding is selected, however this may lead to an increased rate of retransmissions. Dr can, in turn, be reduced if the transmission power is increased. Thus, an optimal balance between Dt and Dr is desirable.
Accordingly, it would be desirable to provide systems and methods for providing enhanced power control in wireless packet data systems.
The present invention overcomes the above-identified deficiencies in the art by providing a method and system for controlling transmit powers based upon time parameters associated with wireless packet data systems. According to one exemplary embodiment of the present invention, downlink transmit power is adapted based upon a queue time of a data packet. For example, as the queue time of a particular data packet stored in a buffer approaches a threshold time, e.g., that specified by a user""s subscription for a particular connection, the transmit power for that packet (as well as other packets associated with that connection) can be increased to reduce the remaining delay associated with receiving that packet at the other end of the connection. This results in a prioritization of the transmission of the data packet and, therefore, a reduction in the delay associated with retransmission.
According to another exemplary embodiment of the present invention, link adaptation can also be adjusted based upon this prioritization. For example, if a data packet is prioritized by providing increased transmit power, then the modulation and/or error correction coding scheme used to process the data packet for transmission can also be changed, e.g., to increase throughput.